Hi Doug:
> Generally speaking, in the US, one may not use a fuse value of more than
> 125% of the rated current of a product.
No.
The value for a fuse (used within an equipment) is
selected based on the value of fault current, not
based on the value of the rated current. This is
because the fuse only operates under fault
conditions.
The presumption is that, without the fuse, the
fault condition would result in a fire or other
hazard. The fuse value is a value that will
operate when that or a similar fault occurs.
Or, putting it another way, put in a fuse of any
value. Perform the fault tests. If the fault
causes the fuse to operate, then the test has an
acceptable result, and the fuse value is
acceptable. If the fault test causes a hazardous
condition and the fuse does not operate, then the
fuse value is too large.
> My problem is, I've already had one agency tell me I can't do this.
Ask the agency to point to the requirement in the
standard that specifies the value to be no more
than 125% of rated current. I'll buy you a beer
if they can do this!
As a general rule, the fuse value should be selected
such that there will be no nuisance operation. Such
nuisance operation often results in the user (or
someone) replacing the fuse with a larger value --
which may allow a hazardous condition to arise!
Use engineering to select the fuse value, not
"jawbone" assertions of an agency.
("Jawbone" is the term used to describe the actions
of Nixon-presidency administrators who expressed
their self-made rules as laws in order to get their
way, and to get around Congress.)
Best wishes for the holiday season,
Rich
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